The present invention relates to novel charge modified chelators useful to attach radioactive metal ions to tumor selective antibodies.
Functionalized chelants, or bifunctional coordinators, are known to be capable of being covalently attached to an antibody having specificity for cancer or tumor cell epitopes or antigens. Radionuclide complexes of such antibody/chelant conjugates are useful in diagnostic and/or therapeutic applications as a means of conveying the radionuclide to a cancer or tumor cell. See, for example, Meares et al., Anal. Biochem., 124, 68-78 (1984); and Krejcarek et al., Biochem and Biophys. Res. Comm., 77, 581-585 (1977).
Attachment of the metal ion to a protein such as an antibody has been achieved by complexation by an acyclic chelate such as carboxymethylated amine derivatives of ethylenediaminetetraacetic acid (EDTA) (U.S. Pat. No. 4,662,420) or diethylenetriaminepentaacetic acid (DTPA) (U.S. Pat. Nos. 4,479,930 and 4,454,106), covalently linked to the antibody. Such acyclic complexes, however, tend to be unstable in vivo either as a result of acid-catalyzed decomplexation or competitive chelate binding by calcium or zinc in serum. The lack of stability can result in uncomplexed metal atoms in the body which have a cytotoxic effect on healthy tissue (e.g., bone marrow).
An alternative to the use of acyclic chelates for the labeling of antibodies is the use of macrocyclic ligands. Certain macrocyclic bifunctional chelating agents and the use of their copper chelate conjugates for diagnostic or therapeutic applications have been disclosed in U.S. Pat. No. 4,678,667 and by Moi et al., Inorg. Chem., 26, 3458.congruent.3463 (1987). Attachment of the aminocarboxylic acid functionality to the rest of the bifunctional chelating molecule is through a ring carbon of the cyclic polyamine backbone. Thus, a linker attached at one end to a ring carbon of the cyclic polyamine, is also attached at its other end to a functional group capable of reacting with the protein.
Tetraazamacrocycles which can act as bifunctional chelating agents for use in diagnosis and therapy are described in published European Patent Application 0 382 582, on Aug. 16, 1990. At least one of the side arms emanating from an amine of the ring being a phosphinate ester. The linker group being attached to a ring carbon of the tetraazamacrocycle.
U.S. Pat. No. 4,994,560 discloses rhodium complexed to bifunctional chelators containing a polyamine, wherein the polyamine chelating agent can be a polyazamacrocycle, where the linker group is covalently attached to any one of the carbon or nitrogen atoms of the polyamine. Such polyamines form outstandingly inert complexes with rhodium (III), but are inappropriate for the radiochelation of lanthanide (III) metal ions.
Carboxymethylated tetraazamacrocycles have been described which are capable of forming inert complexes with copper and lanthanide metal ions and can be conjugated to antibodies [WO 87/050309 WO 89/01476 and Moi et al., J. Am Chem. Soc., 110, 6266 (1988)]. Published PCT Application WO 89/02788 describes bifunctional chelators wherein a number of the secondary amine groups are carboxymethylated.
The present invention provides carboxymethylated tetrazamacrocycles, wherein one or two of the carboxylates are replaced with carboxamides. These chelating agents are antibody conjugatable and form complexes with lanthanides and other substitutionally labile metal ions which are kinetically inert. Upon metabolism of the antibody conjugate, these chelates exhibit rapid whole body clearance, and in certain cases have unexpectedly lower propensity for uptake in bone. Lower bone uptake would be expected to result in a less toxic radiopharmaceutical. The chelates also rapidly form chelate-protein conjugates.